In the past two decades, synthesis of responsive membranes with pores that could be opened and closed by changing chemical or physical properties of their environment has been the subject of many publications (see Mathias Ulbricht's work cited below and hereby incorporated herein by reference for a recent review). See Altinkaya, S. A., Yenal, H., and Ozbas, B.; Apel, P.; Yan, F., and Goedel, W. A.; Yan, F.; Goedel, W. A.; Jiang, P., Hwang, K. S., Mittleman, D. M., Bertone, J. F., and Colvin, V. L.; Cassagneau, T., and Caruso F., Park, S. H. and Xia, Y. N.; and Jiang, P., Bertone, J. F. and Colvin, V. L.; all hereby incorporated herein by reference. In most studies the variable pore permeability was attained by the surface modification of commercial microfiltration membranes using polymers that expand or contract in response to external stimuli. Radiation and plasma-induced graft-polymerizations are employed to immobilize a monolayer of surface-attached responsive polymer chains (brush-like layers) or cross-linked polymer networks (gels) on the membrane and/or pore surfaces. Stimuli-induced changes of the conformation of the grafted polymer chains affected the permeability of nanometer-sized pores in the membranes. Responsive gels are used to fill the interior of larger submicron/micron pores and regulate the membrane permeability. Membranes sensitive to changes of temperature, pH, ionic strength, light intensity, red-ox state of functional groups, and concentration of various substances have thus been fabricated based on the above-mentioned principles. The application of such stimulus-responsive membranes or “chemical valves” (functional gates) includes flow control, size-selective filtration, chemical and bioseparation, controlled release of chemical substances and drugs, and chemical sensors.
Microporous polymer membranes (pore size 0.01-20 μm) are widely used in industry, medicine, pharmacology, and research for separation and concentration of particles, colloids, proteins, enzymes, and cells. Phase inversion and track etching are well-established, commercially implemented techniques for the fabrication of such membranes. In the phase inversion technique, a solvent for a polymer turns into a non-solvent causing the polymer precipitation; the non-solvent serves as a porogen that evaporates after the membrane formation. Phase inversion is usually achieved by immersion of a solution film into a coagulation bath with a non-solvent (immersion precipitation), by exposure to non-solvent vapor, or by temperature change (temperature-induced phase separation). Membranes prepared by this technique exhibit a highly porous inner structure represented by a continuous network of interconnected tortuous pores. Furthermore, the structure is usually asymmetric with a thin dense surface layer and a thick sponge-like basic layer. The surface layer determines the separation properties and the overall flow resistance of the membrane, while the basic layer acts as a mechanical support. See Altinkaya, S. A.; Yenal, H.; Ozbas, B. The track etch (TE) membranes are prepared using a two-step fabrication procedure. See Apel, P. First, a polymer film (polycarbonate or polyester) is exposed to a collimated beam of heavy ions that produce parallel tracks across the film. The tracks are then chemically etched forming cylindrical pores. Unlike the phase inversion membranes, the track etch membranes are characterized by uniform pore size and relatively low pore density resulting in the high flow resistance.
In addition to the above mentioned techniques, various template-assisted methods for the fabrication of microporous polymer membranes have been reported in the literature. Colloidal crystals (See Yan, F.; Goedel, W. A. (both); Jiang, P., Hwang, K. S., Mittleman, D. M., and Bertone, J. F., Colvin, V. L.; Cassagneau, T.; Caruso, F., Park, S. H., Xia, Y. N., and Jiang, P., Bertone, J. F., Colvin V. L.) and emulsion droplets (See Imhof, A.; Pine, D. J. hereby incorporated herein by reference) have been successfully implemented as templates for the fabrication of self-standing membranes. Microcontact molding See Odom, T. W., Love, J. C., Wolfe, D. B., Paul, K. E., and Whitesides, G. M. hereby incorporated herein by reference.) and casting of polymer layers on solid substrates with 2D pillar arrays (See Yan, X. H., Liu, G. J., Dickey, M., and Willson, C. G. hereby incorporated herein by reference.) was found to be feasible for the fabrication of thin film membranes. An elegant method for the fabrication of membranes with regular pore arrangement is based on controlled drying of a polymer solution cast as a layer on a solid substrate in a highly humid atmosphere. See Widawski, G., Rawiso, M., and Francois, B.; Yabu, H., Tanaka, M., Ijiro, K., and Shimomura, M.; Maruyama, N., Koito, T., Nishida, J., Sawadaishi, T., Cieren, X., Ijiro, K., Karthaus, O., and Shimomura, M.; Karthaus, O., Maruyama, N., Cieren, X., Shimomura, M., Hasegawa, H., Hashimoto, T.; and Srinivasarao, M., Collings, D., Philips, A.; and Patel, S. hereby incorporated herein by reference. Evaporation of a water-immiscible solvent leads to cooling of the solution surface and, consequently, to condensation of a monolayer of hexagonally arranged water droplets that gradually sink into the solution. As the solvent evaporates and the polymer vitrifies, the droplets evaporate leaving pores behind. Polyelectrolyte layer-by-layer assembly (See Li, Q., Quinn, J. F., Caruso, F. hereby incorporated herein by reference.) and self-assembly of hollow micelles from rod-coil block copolymer represent another, non-template strategy for the fabrication of microporous layers. See Jenekhe, S. A., and Chen, X. L. and Ulbricht, M. both hereby incorporated herein by reference.
Over the past decade, numerous studies are devoted to the fabrication of membranes with variable permeability; specifically, the membranes which pore size can change in response to external stimuli (responsive membranes). Such membranes are of potential interest for a variety of applications, e.g., flow regulation, size- and charge-selective filtration and fractionation, encapsulation of living cells, controlled drug release, and sensors. The common approach for the fabrication of responsive membranes consists of the surface modification (e.g., by graft polymerization) of a premade membrane (usually commercial TE and anodized aluminum membranes as well as membranes fabricated by the phase inversion techniques) with a layer of stimuli-sensitive polymer chains (brush) or a stimuli-sensitive cross-linked polymer network (gel). See Osada, Y., Honda, K., and Ohta, M.; Kontturi, K., Mafe, S., Manzanares, J. A., Svarfvar, B. L., and Viinikka; Mika, A. M., Childs, R. F., Dickson, J. M., Mccarry, B. E., and Gagnon, D. R.; Ito, Y., Park, Y. S., and Imanishi, Y.; Iwata, H., Hirata, I., and Ikada, Y.; Peng, T., Cheng, Y. L.; and Ito, T., Hioki, T., Yamaguchi, T., Shinbo, T., Nakao, S., and Kimura, S.; Iwata, H., Oodate, M., Uyama, Y., Amemiya, H., and Ikada, Y.; Park, Y. S., Ito, Y., and Imanishi, Y.; and Yamaguchi, T., Ito, T., Sato, T., Shinbo, T., Nakao, S. all hereby incorporated herein by reference. In a few studies,30-33 (See Ying, L., Wang, P., Kang, E. T., and Neoh, K. G.; Liu, G. J.; Lu, Z. H., and Duncan, S.; Zhai, G. Q., Toh, S. C., Tan, W. L., Kang, E. T., Neoh, K. G., Huang, C. C., and Liaw, D. J.; and Hester, J. F., Olugebefola, S. C., and Mayes, A. M. all hereby incorporated herein by reference.) responsive membranes are prepared in one step by the immersion precipitation of graft-copolymers with stimuli-responsive side chains. The main requirement is that the thickness of the stimuli-sensitive layer has to be comparable with the pore size. Then, the pore permeability will depend greatly on the conformational state of the chains or on the swelling degree of the gel. Responsive membranes in which permeability was dependent on pH (See Kontturi, K., Mafe, S., Manzanares, J. A., Svarfvar, B. L., and Viinikka, P.; Mika, A. M., Childs, R. F., Dickson, J. M., Mccarry, B. E., and Gagnon, D. R.; Ito, Y., Park, Y. S., and Imanishi, Y.; Iwata, H., Hirata, I., and Ikada, Y.; and Peng, T., and Cheng, Y. L. Ying, L.; Wang, P.; Kang, E. T., Neoh, K. G., Liu, G. J., Lu, Z. H., and Duncan, S.; and Hester, J. F., Olugebefola, S. C., and Mayes, A. M.: all hereby incorporated herein by reference.) and/or ionic strength, Kontturi, K., Mafe, S., Manzanares, J. A., Svarfvar, B. L., and Viinikka, P. and Zhai, G. Q., Toh, S. C., Tan, W. L., Kang, E. T., Neoh, K. G., Huang, C. C., and Liaw, D. J.) presence of specific ions,26,31 (See Ito, T.; Hioki, T., Yamaguchi, T., Shinbo, T., Nakao, S., Kimura, S.; and Zhai, G. Q., Toh, S. C., Tan, W. L., Kang, E. T., Neoh, K. G., Huang, C. C., and Liaw, D. J.) temperature, (See Peng, T., Cheng, Y. L., Ito, T., Hioki, T., Yamaguchi, T., Shinbo, T., Nakao, S., and Kimura, S.; Iwata, H., Oodate, M., Uyama, Y., Amemiya, H., Ikada, Y., and Park, Y. S., Ito, Y., and Imanishi, Y. all hereby incorporated herein by reference.) and the strength of an applied electric field (See Ly, Y., and Cheng, Y. L. hereby incorporated herein by reference.) have been reported.
Recently, we reported a novel method for the fabrication of responsive polymer gel membranes. See Tokarev, I., Orlov, M., and Minko, S. hereby incorporated herein by reference. The membranes are deposited as thin films on solid substrates from a solution containing poly (2-vinylpyridine) (P2VP) and 1,4-diodobutane (DIB). It is noteworthy that DM had a two-fold function: it served as a porogen and as a cross-linker for P2VP. Because P2VP is a weak cationic polyelectrolyte, its cross-linked network (gel) demonstrates pH-dependent swelling properties. We observed the reversible contraction of the pores of membranes subjected to acidic water, which was caused by swelling of the P2VP gel. Unlike, the responsive membranes previously reported in the literature, the P2VP gel membranes change the pore size by expansion/contraction of their entire body. We demonstrated that the thin film membranes could be easily transferred onto a surface of commercial track etch membranes to produce hybrid membranes with pH-controlled water permeability.
In this application, we focus on aspects related to the fabrication of the P2VP gel membranes. In particular, we discuss the possible mechanism of the pore formation and demonstrate that humidity is essential for the formation of pores in the P2VP layers.
Prior to this disclosure a membrane body either was not permeable and responsive layer was on the wall of the pores. The possibility to regulate an upper limit of pore size was limited by the size diameter of pores in the rigid matrix. In permeable gels, the possibility to regulate pore size was limited to the distance between cross linking points in the network.
The object of the present invention is concerned with a stimuli-responsive (SR) polymer membrane and method of making the same. The method of making the membrane is new. The entire body of the responsive membrane is a gel. Gels are used as membranes because they are permeable-swollen network. This disclosure discusses a new combination of cylindrical pores in a swollen network. When the network swells or shrinks the cylindrical pores open or close. Thus, inside the network, one can introduce ligands, function groups which due to specific interaction with some signaling molecules in the surrounding environment can cause swelling or shrinking the membrane and this way they open or close pores.
With cylindrical pores in a gel there is the ability to regulate pore size in a broad range and an ability to arrange response by adding some functional groups inside the gel body.